Plywood is manufactured by assembling multiple layers of veneer and glue in a sandwich that is compressed and glued together to form the sheet of plywood. Typically, face sheet laminations, for example, that vary in thickness, such as from 1/16 inch to ¼ inch, are positioned as the outermost layers of the plywood. Knots and other voids in these face sheets are patched to upgrade the face sheets to improve the overall grade of the plywood. These face sheets are typically generally rectangular in shape, although it is not uncommon for veneer sheets to have non-orthogonal corners that are produced during manufacture of the sheets. Typically, face sheets are oversized to allow trimming of the assembled plywood sheet to a standard dimension with orthogonal corners. For example, face sheets used to produce 4-ft.×8-ft. standard sheets of plywood may be 51 in.×102 in., or otherwise oversized in dimension, to allow for such trimming. The face sheets need not be of a single piece of material, as face sheets or other veneer sheets can be formed by edge gluing elongated strips together to form the face sheets.
The surface roughness of plywood can be used for grading or other sorting or for manufacturing process control. While surface roughness estimation is important in assessing plywood, surface roughness estimation is also important in assessing materials in many different industries. Unfortunately, conventional methods of roughness assessment tend to be subjective, labor-intensive, expensive, inaccurate, or disruptive of manufacturing process flows. Surface roughness measurements of wood veneer based on light shadows produced by illuminating a surface under test with illumination at near grazing incidence tend to confuse surface ripples or differences in surface reflectivity caused by dark defects with roughness. 3-D cameras can be used but the height differences associated with veneer roughness are typically small in comparison to height differences readily detected by a 3-D camera. For these and other reasons, improved surface roughness estimation methods and apparatus are needed.